The field of invention relates to cyclic ester oligomers (CEOs), and in particular, to CEOs manufactured from linear polyesters or their precursors.
Cyclic ester oligomers (CEOs) have been known for a long time, see for instance U.S. Pat. No. 2,020,298. They are known to be present in varying, usually small, quatities in many linear polyesters and have been isolated from such linear polyesters. They are often low viscosity liquids, and it has been known for a long time that they may be polymerized to higher molecular weight linear polyesters by ring opening polymerization, see for instance U.S. Pat. Nos. 5,466,744 and 5,661,214 and refeences cited therein. This ability to readily form a high molecular weight polymer from a relatively low viscosity liquid has made these CEOs attractive as materials for reaction injection molding type processes, wherein a low viscosity material is converted to a high molecular polymer in a mold, so that a final shaped part is obtained.
However, such CEOs have been difficult and expensive to prepare, for example, requiring very high dilution conditions and/or using relatively expensive starting materials such as diacyl halides in conjunction with diols and a base to react with the HCl formed, see for instance U.S. Pat. No. 5,466,744. Therefore, lower cost routes to CEOs are of great interest in view of these high manufacturing costs.
In a paper concerned with the synthesis of CEOs, H. R. Kricheldorf, et al., Macromolecules, vol. 34, p. 713-722 (2001) (herein Kricheldorf), two types of situations for the synthesis of CEOs are described: a kinetically controlled scenario and an equilibrium controlled scenario. Kircheldorf shows that in a kinetically controlled scenario nearly quantitative yields of CEOs can be obtained in concentrated solutions if certain ideal conditions are met (it appears the conditions were similar in U.S. Pat. Nos. 5,466,744 and 5,214,158) in the process. Unfortunately, relatively expensive ingredients are used, and such ideal conditions are difficult to achieve in commercial processes. As Kircheldorf points out, the process he uses is kinetically controlled.
Most (linear) polyester manufacturing, however, is carried out under equilibrium conditions, such as melt polymerization and/or solid state polymerization. When preparing CEOs under such conditions, for example from linear polyesters, high dilution is required, see for instance U.S. Pat. No. 5,407,984, in which small amounts of linear polyester are converted in highly dilute solution to CEOs. All of this chemistry depends upon the fact that various ester linkages are, under conditions where the reaction rates are high enough, labile and in equilibrium with one another.
One can represent a polyester made from a diol (D) and a dicarboxylic acid (A) by the formula xe2x80x94(AD)nxe2x80x94 (I) where the end groups are unspecified. The ester groups within (I) and between molecules of (I), as well as the hydroxyl and carboxyl groups in (I) (the end groups) are, under the proper conditions (usually elevated temperatures and optionally in the presence of esterification/transesterification catalysts), in equilibrium with one another. This chemistry forms the basis for the melt and solid state polymerizations to form linear polyesters. For instance: 
It is evident that by repetition of equations (2) and (3) the molecular weight of the polymer being formed may be built up, particularly if the equilibrium is forced to the right in each instance by removal of the byproduct water. It is also evident that the product of equation (2) may cyclize to the cyclic ester dimer according to equation (4). 
Equation (4) illustrates one way in which the CEOs can be in equilibrium with the linear polyesters (there are other ways, see below). Although equation (4) shows a CEO being formed by an esterification, they can also be formed by transesterification. The linear polyester molecular weight may also be built up by reactions such as 
All of the reactions shown so far are esterifications, and any of the ester groups within these molecules may undergo ester exchange also, and as one can see, complex equilibria between various species may exist. However so long as water (or alcohol or diol if esters are part of the starting materials) is removed from the system, which is usually accomplished by the application of heat and sometimes vacuum, the molecular weight of resulting polymer will be increased. As it turns out, the equilibrium between linear and cyclic esters in this type of system usually favors the linear polyesters in concentrated conditions. As conditions become more dilute in the polyester (cyclic or linear) the equilibrium shifts in favor of the cyclic esters. Thus in polyesters produced by a melt process or a so-called solid state process the polyester is concentrated (usually only polyester plus a small amount of catalyst is present), so that while CEOs are present in the product of such processes, they are usually present in small amounts.
Another type of reaction that may be important for the formation of CEOs from linear esters, and the formation of linear polyesters from CEOs is thought to be 
A similar reaction could be written using an end hydroxyl group. All of the processes for making CEOs from linear polyesters until now have simply heated the linear polyesters under very dilute conditions so that more cyclics were formed in the reactor.
In all of equations (2)-(6) the choice of end group, and in some cases which end groups reacts, hydroxyl or carboxyl, is somewhat arbitrary, and other combinations are clearly possible and likely.
All of the above esterification/transesterification reactions may be speeded up by the addition of esterification/transesterification catalysts, although it is important to realize that certain catalysts may favor polymerization, depolymerization, linears and cyclics, or combinations of these.
This invention includes processes for the production of cyclic ester oligomers, comprising, subjecting a linear polyester to a continuous reactive extraction at a temperature at which said linear polyester is molten and which is sufficient to cause formation of said cyclic ester oligomers, wherein at least some of said linear polyester is converted to one or more cyclic ester oligomers and said extraction is carried out by a fluid which is one or both of a liquid and a gas to separate said cyclic ester oligomers from said linear polyester.